This invention relates to the vapor phase growth of semiconductor material such as mercury-cadmium-telleride (HgCdTe) epitaxial layers as in the construction of a semiconductor structure of an infrared radiation detector and, more particularly, to a method for causing the vapor phase growth of HgCdTe epitaxial layers within a quasi-open growth system.
HgCdTe semiconductor material is commonly employed in the construction of high performance radiation detectors. Typically, HgCdTe detectors are utilized for the detection of radiation in the infrared portion of the electromagnetic spectrum.
A typical HgCdTe detector is comprised of a CdTe base layer, which is transparent to the radiation to be detected, and an overlying layer of radiation absorbing HgCdTe. The HgCdTe layer is typically doped to have the characteristics of a p-type semi-conductor material. Within an upper surface of the HgCdTe layer a number of n-type junctions are implanted typically in a regular array. The interface of the n-type junctions and the p-type HgCdTe layer forms an array of photodiodes. When the foregoing radiation impinges upon a lower surface of the transparent base layer, the radiation then passes through the base layer and into the HgCdTe layer. Each photodiode of the array generates a current which is proportional to the radiation absorbed within the adjacent semiconductor material of the HgCdTe layer. This current is detected by a suitable means, typically an integrated circuit multiplexer, connected to the array for scanning the individual photodiodes thereof, whereby an image corresponding to the impinging radiation is formed.
The relative proportions of mercury to cadmium in the aforesaid radiation absorbing layer typically determines the energy bandgap of the layer and hence the range of wavelengths which will be absorbed therein. The relative proportions of Hg and Cd are indicated typically by expressing the compositional relationship in the form of Hg.sub.1-x Cd.sub.x Te. For example, if x is approximately 0.3, the radiation absorbing layer will be most sensitive to radiation having a wavelength range of approximately 2.0 to 3.5 microns. This range of wavelengths is associated typically with short wavelength infrared radiation, commonly abreviated as SWIR.
As may well be appreciated, it is a natural goal in the design of a high performance Hg.sub.1-x Cd.sub.x Te radiation detecting array that the individual photodiodes of the array have essentially identical electrical and optical characteristics. This desired uniformity of photodiodes ensures that the resultant image will faithfully reproduce the relative intensities of the radiation incident upon the array. The surface morphology and compositional uniformity of the Hg.sub.1-x Cd.sub.x Te radiation absorbing layer, among other factors, are important considerations in the design and fabrication of a high performance radiation detecting array.
It has been well known in the art to form the radiation absorbing layer by the method of liquid phase epitaxy (LPE), wherein a layer of HgCdTe is grown upon the surface of a CdTe substrate by contacting the surface of the CdTe substrate with a melt of HgCdTe. A portion of the HgCdTe is deposited upon the surface of the CdTe substrate forming a layer of HgCdTe. Typically, the LPE process is carried out by placing the constituient components in an open, or unsealed, container which is placed within a furnace, the furnace generating sufficient heat to provide the liquid phase. The LPE method, while suitable for forming a HgCdTe radiation absorbing epilayer, may also be disadvantageous for several reasons.
One disadvantage of the LPE method is that the surface morphology may not be optimum, due to the difficulty of controlling the growth of the HgCdTe epilayer. Another disadvantage of the LPE method is that there is a potential for non-uniformity in both the composition and thickness of the resultant HgCdTe epilayer. One further disadvantage of the LPE method is that inclusions from the HgCdTe melt may be deposited in the epilayer, resulting in surface defects which may prevent the epilayer from being useable for forming a regular array of photodiodes.
In response to the above mentioned disadvantages inherent in the LPE method, a vapor phase epitaxy (VPE) method has been utilized in the prior art to form the HgCdTe radiation absorbing epilayer. This method employs the vaporization of a HgTe source material, and the subsequent deposition of this vapor upon the surface of a CdTe substrate wafer.
This prior art VPE method was carried out, typically, in a sealed quartz ampoule wherein a CdTe wafer and a quantity of HgTe source material were placed. The ampoule was then exposed to heat sufficient to vaporize the source material, the vapor thus produced being deposited upon the surface of the wafer as a thin epitaxial layer. Diffusion of Cd from the wafer into the epitaxial layer results in the formation of a HgCdTe epilayer upon the CdTe wafer.
While this VPE method may be utilized to form a HgCdTe epilayer on the surface of a CdTe substrate, it has several disadvantages associated with its use. One major disadvantage is that, because the ampoule is sealed, very little control over the VPE deposition process is possible. Thus, the composition of the epilayer may be non-uniform, the uniformity being affected by the ratios of the total ampoule volume to source volume, the surface area of the substrate, and other factors. The sealed quartz ampoule method of VPE is particularly disadvantageous in a production environment, due to the expensive and time consuming operations involved in fabricating, sealing, and opening the ampoules. Practical considerations relating to the maximum size of a quartz ampoule place restrictions on the maximum wafer size which may be contained therein, resulting in a further disadvantage that radiation detectors having a large surface area may not be readily fabricated by the sealed ampoule method of the prior art.